Numerous combinations of oils, graphite, soot, diatomaceous earth, silica, organic binders, etc., have been used to protect metallic casting belts or to insulate them and/or to act as parting agents to prevent adherence to the belts in continuous casting machines for casting molten metal. Such prior coatings are temporary or transitory in nature and may be continually applied and replenished during casting. The continual application of such coatings while casting requires precise maintenance and control in view of the need for consistent thermal conductivity. This continual application and replenishing of temporary insulative coatings is a difficult and imprecise art. For example, excess liquid or solvent or binder in the insulating coating material is likely to emanate gas in such quantity as to disturb the soundness of the cast product, resulting in porosity. Some of the gas thus liberated is at times hydrogen, which can detrimentally alter the metallurgical qualities of the cast metal. Also excess amounts of the temporary insulative coating material itself may accumulate near the edges of the cast product and usurp part of the continuously moving mold space, causing defects in the cast product.
A two-layer belt coating, including thermosetting resin and solvent, for use in continuously casting relatively low melting-point metals, such as aluminum, zinc and lead is described in U.S. Pat. No. 3,871,905. Coatings containing resins are generally unsuitable for use for continuously casting metals having melting-point temperatures significantly higher than aluminum.
A casting belt made of mild killed steel containing 0.2% to 0.8% by weight of titanium has been multiple-laver coated, as described in U.S. Pat. No. 4,298,053. The surface of the belt is first coated by a "primer" layer of a nickel-aluminum alloy (80% by wt. of Ni and 20% by wt. of Al) stated to be 0.005 mm thick in the specification but claimed to be 0.05 mm thick in the only claim. This primer layer is coated by another layer between 0.01 and 0.5 mm thick made of chromium, or of an alloy of chromium, or of nickel, or of an alloy of nickel or of a stainless steel. Then, a third layer of colloidal graphite anti-adhesion agent is applied over the second layer. However, in our experience more thermal insulation and additional non-wettability are required than can be obtained by following the teaching of that patent.
Canadian Pat. No. 1,062,877 of Thym and Gyongyos describes the coating of endless casting belts by several thin layers (80-100 micrometers, preferably 50-70 micrometers) on the endless casting belts until the desired thickness of ceramic layers is achieved to give the requisite thermal resistance. Such a build-up of multiple ceramic layers is laborious, time-consuming and expensive. The resulting built-up coating is machined mechanically, e.g. by grinding, in order to achieve the desired uniform surface finish and wetting behavior between this multiple-layer ceramic coating and the aluminum being cast. This built-up ceramic coating consists of Al.sub.2 O.sub.3, CaZrO.sub.3, Al.sub.2 O.sub.3.MgO, ZrSiO.sub.4 or Al.sub.2 O.sub.3.TiO.sub.2. It is built up in thickness until it provides thermal resistance in the range of 10.sup.-4 to 10.sup.-3 m.sup.2.h. .degree.C/kcal.
Such built-up ceramic coatings are usually relatively thick and relatively fragile and brittle. They have insufficient durability to withstand thermal shock, or to withstand the mechanical stretching and relaxing, the flexing and abrading which are inherent in continuous casting employing one or more moving belts as molten-metal-contacting-cooling surfaces.
Durability to withstand such mechanical and thermal stresses are important, as otherwise bits of the ceramic coating become loose and spall during the demanding service imposed upon them in continuous casting of molten metals. The loosened bits inevitably become inclusions in the cast metal product. Such inclusions can become a serious problem, as for instance in the case of copper destined for drawing into fine wire. Such inclusions cause the wire to break in the dies, resulting in significant productivity losses as the wire is restrung. Ceramic coatings are generally not flexible and tend to be fragile.
Problems associated with brittleness, ceramic flake-off and contamination of the cast product by ceramic particles are highlighted in German Pat. No. 24 11 448 of Theobald, in which patent an attempt was made to solve this problem when casting aluminum by applying over the relatively thick ceramic a second and protective abrasion resistive metal layer which has a higher temperature point of fusion than the metal to be cast.